Download Soil

Monday, Oct. 3
1. Field trip A1 & A2 signups: make sure you are where you
think you should be…
2. You will get a ZERO for the field trip if you do not
participate
Soils
We know more about the movement of celestial bodies
than about the soil underfoot.
- Leonardo da Vinci
Soil: Definition
• Solid earth material that has been altered
by physical, chemical and organic processes
so that it can support rooted plant life.
• Engineering definition: Anything that can be
removed without blasting
Soil Production
Soil Production: Inputs
Conversion of rock to soil
Soil Production: Outputs
Downslope movement of soil
Soil Thickness: Storage
input ± output = soil thickness
or: rock conversion ± soil transport = thickness
that is, soil thickness reflects the balance
between rates of soil production and rates of
downslope soil movement.
• Slope
• Weathering Rate
Factors of Soil Formation
•
Climate
•
Organisms
•
Parental Material
•
Topography
•
Time
Factors of Soil Formation
Climate
•
Temperature and
precipitation
•
Indirect controls (e.g.,
types of plants)
•
Weathering rates
The greater the rainfall
amount, the more rapid
the rate of both
weathering and erosion.
Factors of Soil Formation
Organisms
•
Types of native vegetation
•
Weathering is dependent of plant growth
•
Plant and animal activity produces humic acids that
are powerful weathering agents. [acids derived from
chemical breakdown of organic matter]
•
Plants can physically as well as chemically break
down rocks.
•
Plants stabilize soil profiles, Animals (including
humans) tend to increase erosion.
Factors of Soil Formation
Parent Material
• Chemistry
• Mineralogy
• Grain size
Factors of Soil Formation
Topography:
• Ground slope
• Elevation
• Aspect (north vs. south facing slopes)
Factors of Soil Formation
Downslope transport of soil
is a function of slope:
Erosion rate = f(S)
Steeper slopes erode faster.
The steeper the surface
slope, the more likely any
eroded material is to be
transported out of the
system.
Factors of Soil Formation
Soils on hillslopes reach an equilibrium thickness, often
about 1m.
Soils on flat surfaces, such as floodplains or plateaus,
tend to thicken through time due to weathering rates
being greater than sediment transport rates.
Factors of Soil Formation
Time for development and destruction of soil
profiles
Typical chemical reaction rates are slow 
the longer a rock unit has been exposed,
the more likely it is to be weathered
And, the longer soil waits before transport,
the thicker it can become…
Processes of Soil Development
combined effects of:
• additions to ground
surface
• chemical
transformations
• vertical transfers
• removals from soil
relative importance
varies
Additions to soils
Inputs from outside ecosystem
Atmospheric inputs
Precipitation, dust, deposition
Horizontal inputs
Floods, tidal exchange, erosion, landwater movement
Inputs from within ecosystem
Litterfall and root turnover
Transformations
Decomposition of organic matter
Breakdown to form soluble compounds that
can be absorbed leached away
Depends on input quantity, location (roots,
leaves), environment (temp & precip)
Humification to form complex organic matter
Weathering of rocks
Physical weathering / fragmentation of rock
Freeze-thaw; drying-wetting; fire
Chemical weathering
primary  secondary minerals
Parent material (bedrock) undergoes weathering to
become regolith (soil + saprolite).
Soil is a mixture of mineral and organic matter
lacking any inherited rock structure.
Soil
Saprolite is weathered rock that retains remnant
rock structure.
Saprolite
Saprolite
Soil Horizons and Profiles
Soil Horizons
over time, soil layers differentiate into distinct
‘horizons’
not deposited, but zones of chemical action
• Chemical reactions and formation of secondary
minerals (clays).
• Leaching by infiltrating water (elluviation)
• Deposition and accumulation of material leached
from higher levels in the soil (illuviation)
Soil Profile
Suite of horizons at a given locality
Typical soil profile
Cookport soil,
Pennsylvania
A Horizon
B Horizon
C Horizon
Soil classification = messy
Soil classification = soil orders
Aridisols
=
Mollisols =
Alfisols, Ultisols,
Spodosols =
Oxisols
=
Histosols =
Gelisols
=
Andosols =
Vertisols =
Entisols
=
Inceptisols =
arid zone soils (calcic horizons)
grassland soils (thick A horizon)
and
forest soils (thick B horizon)
tropical soils (quite oxidized)
wetland soils
polar soils
volcanic parent material
swelling clays
weak A over C horizon
weak B horizon
Soil classification = soil orders
Aridisols
=
Mollisols =
Alfisols, Ultisols,
Spodosols =
Oxisols
=
Histosols =
Gelisols
=
Andosols =
Vertisols =
Entisols
=
Inceptisols =
arid zone soils (calcic horizons)
grassland soils (thick A horizon)
and
forest soils (thick B horizon)
tropical soils (quite oxidized)
wetland soils
polar soils
volcanic parent material
swelling clays
weak A over C horizon
weak B horizon
Soil types (more simply) – Aridisols
Physical weathering breaks rocks into small mineral particles.
Soil types – Oxisols
Chemical weathering dissolves and changes minerals
at the Earth’s surface.
Soil types – Mollisols
Decomposing organic material from plants and
animals mixes with accumulated soil minerals.
Limits on soil development
Balance Between:
Downward Lowering of Ground Surface
Downward Migration of Soil Horizons
If erosion rapid or soil evolution slow, soils may
never mature beyond a certain point
Extremely ancient soils may have lost everything
movable
Rates of Soil Development
U.S. Department of Agriculture estimates that it
takes 500 years to form an inch of topsoil.
That’s less than 0.01 mm yr-1
Modern rates of soil loss are 100 to 1000 times
rates of soil formation (typically > 1 mm yr-1 in
agricultural settings).
Sets up a fundamental problem due to the erosion
of natural capital!
Soil and the Life-Cycle of Civilizations
How long would it take to erode a 1m-thick soil?
Thickness of soil divided by the difference between
Rate of soil production and erosion.
1m
≈ 1000 years
1 mm/yr-1- .01 mm/yr-1
This is about the life-span of most major civilizations...
Man—despite his artistic pretensions, his
sophistication, and his many accomplishments—owes
his existence to a six-inch layer of topsoil and the fact
that it rains.
- Author Unknown
A nation that destroys its soils, destroys itself.
– President Franklin D. Roosevelt, Feb. 26, 1937.
National Archives: 114 SC 5089
and finally
sign up for field trip A1 or A2 by 5 TODAY
On Friday we will discuss erosion
how to get rid of the materials created by physical &
chemical weathering and soil formation…